This invention relates in general to the field of radar and communications systems, and in particular, to pseudorandom noise coded systems using biphase modulation for autocorrelation. In radar and communications systems, it is sometimes important to process signals having a desired range delay while rejecting signals having an undesired range delay. The autocorrelation performance, also referred to herein as the out-of-range rejection (ORR) performance, is a measure of such a systems ability to accomplish this requirement. Pseudorandom Noise (PN) coded systems are often used to achieve acceptable ORR performance while maintaining low peak transmit power requirements.
Most PN coded systems modulate the PN code onto a carrier frequency prior to transmission, and subsequently demodulate the PN code in the receive path in a correlation process. Conventional pseudorandom noise (PN) systems use a biphase modulator to modulate the characteristics of a maximal length binary code sequence onto the continuously transmitted carrier frequency. The biphase modulator has two phase states which are nominally separated 180 degrees in phase, and which are selected in response to the two output states of the binary code sequence. The phase difference between these two phase states is referred to as the relative phase difference while the amplitude difference between these two phase states is referred to as the relative amplitude ratio. By design, the relative amplitude ratio of the biphase modulator in a conventional PN system is desirably set to one, meaning that the amplitude of the signal at the output of the biphase modulator is desirably equal for the two different phase states. However, in such a conventional PN system, the ORR performance is limited, wherein it is proportional to the square of the PN code length. Thus, for a given PN code length, the ORR performance is fixed.
In designing a PN coded system (radar or communications) there is a system tradeoff to be made between the various waveform parameters such as the PN code length, PN code bit width, Doppler frequency band width, and transmitter center frequency. In a conventional PN system, the PN code length establishes the system's ORR performance, the PN code bit width establishes the system's range resolution, the Doppler frequency band width and the transmitter center frequency establish the maximum closing velocity. Moreover, for a given range resolution, closing velocity and transmitter center frequency, the PN code length and, consequently, the ORR performance is limited. Thus, this system tradeoff often results in a compromise between conflicting system requirements.
Accordingly, it is an aspect of the present invention to provide a significant improvement in the ORR performance of a system while maintaining the same or shorter PN code length. This would provide greater flexibility in selecting the PN modulation waveform parameters, and would allow for the simultaneous improvement of other performance parameters. In a PN radar system, for example, such a means would allow an increase in maximum target engagement velocity and/or an increase in the transmit frequency, while simultaneously improving the ORR performance. It would also provide a capability to detect very small targets in the presence of large out-of-range clutter return using a shorter PN code length.
It is also an aspect of the present invention to method of improving the ORR performance that allows for the adjustment of either the relative phase difference or the relative amplitude ratio, or both, of the two phase states of a biphase modulator. This will provide additional flexibility to more fully optimize the ORR performance.
It is still another aspect of the present invention to control the relative amplitude ratio between the two phase states of a biphase modulator by means of a feedback control loop to maintain a specific desired relative amplitude ratio offset slightly from one, while maintaining the relative phase difference desirably set to 180 degrees. In these applications less complex hardware may be required to demodulate the biphase modulated transmitted signal because phase information would not be necessary for demodulation, while still achieving improved ORR performance.